Amplifier-assisted CRDS (cavity ring-down spectroscopy) toward compact breath sensing

A cavity-ring-down spectroscopy (CRDS) system utilizing waveguide for breath sensing realizes several meter optical path integration that has possibility for ppm-order gas sensing within a compact area. We have proposed an optical amplifier assisted gas sensing system scheme to compensate waveguide propagation loss that may prevent ppm-order gas detection. The amplifier in a CRDS system, however, may result in self-lasing at specific wavelength in case of high pumping condition. Once self-lasing happens, signal light loose its gain from amplifier. The amplifier spontaneous emission that generated by an erbium-doped fiber amplifier affects the background noise level of cavity ring-down waveforms. In this work, we propose the scheme of polarization direction control so as to suspend the self-lasing, and the scheme of additional loss as to suppress ASE power. As a result, we confirmed the suppression of light intensity at a self-lasing wavelength below -50 dBm and the gain improvement of 24 dB at signal.